It is IMPOSSIBLE to tell who is the father in this case. A DNA test is needed to determine paternity.
Ovulating sooner or later can happen to any woman, and you ovulate roughly 14 days before your next period.
its more than likey the guy you had sex with on the 25th 26th March baby. it sounds like you are trying to figure that out as well. on avergae, you usually ovulate on day 14th of your cycle counting start of lmp as day one with a 28 day cycle. however this can vary if your cycle is shorter or longer. however every woman is different and it will be hard for anyone to "guess" when you ovulated or concieved. your best bet is DNA testing. with a 28 day cycle you should have ovualted on or around the 24th of march. if your cycle varies then you may have ovulated sooner or later.
It could really be either guy, because sperm can live inside your body for 5 days, and it is impossible to tell the exact time of conception, either guy could be the father. Just because a guy does not ejaculate, does not mean you didn't get pregnant from him. Pre ejaculate contains sperm. The 19th-25th is only a six day difference, it really could be either. You will need a DNA test to determine the father.